The invention relates to a method of improving the performance of magnesium alloys in respect of microshrinkage.
Magnesium alloys are here understood as being all those which contain from 4 to 10% by weight of aluminium and
either up to 3% of zinc and/or up to 1% of manganese PA1 or up to 1% of silicon and/or up to 1% of manganese, PA1 concentrating microshrinkage in a relatively restricted zone of the piece, and in any case in a zone close to the mouth of the mould, that is to say, the part in the vicinity of the feed, thus enabling a sound piece to be obtained by risering that zone; PA1 very substantially reducing the difference between the minimum density and the density of the alloy where the strontium contents are highest; PA1 thereby improving the mechanical properties of the pieces obtained without harming their corrosion resistance.
the balance being magnesium.
Alloys which the ASTM standards define as follows may be mentioned more particularly:
AZ63 (alloy containing 6.0% by weight of aluminium, 3.0% of zinc, at least 0.15% of magnanese) PA0 AZ80 (alloy containing 8.5% by weight of aluminium, 0.5% of zinc, at least 0.12% of manganese) PA0 AZ91 (alloy containing 8.7% by weight of aluminium, 0.7% of zinc, at least 0.13% of manganese) PA0 AZ92 (alloy containing 9.0% by weight of aluminium, 2.0% of zinc, at least 0.1% of manganese) PA0 AM60 (alloy containing 6.0% by weight of aluminium, 0.13% of manganese) PA0 AM100 (alloy containing 10.0% by weight of aluminium, 0.1% of manganese) PA0 AS41 (alloy containing 4.2% by weight of aluminium, 0.35% of manganese).
These alloys have good mechanical properties and excellent corrosion resistance. However when they are moulded by gravity from liquid metal, either in a sand mould, in a sealed mould or by moulding under pressure, they generally have microshrinkages in their structure. These are due to the fact that the metal contracts during solidification, possibly to the extent of several % by volume. If no liquid metal is added in the contraction zone a void is then produced, resulting in the formation of a cavity or shrinkage.
When the solidification interval of the metal is very long, as in the case of the above-mentioned alloys, a relatively extensive pasty zone forms in the moulded piece, in which contraction takes place gradually. The liquid metal thus has to make its way between the solid dendrites over a great distance and cannot fill the voids. Microcavities are consequently formed, distributed between the grains throughout the pasty zone; these are described as microshrinkages.
Now microshrinkages tend to degrade the mechanical properties of the pieces which contain them. Furthermore, in the case of thin-walled pieces, they form open pores which make them useless for applications where they are subjected to pressure.
When one wishes to obtain moulded pieces from these alloys, which have good mechanical properties or at least are sealed, the problem thus arises of preventing the formation of the microshrinkages without thereby harming other properties such as corrosion resistance.
The problem is not of course new, and persons skilled in the art of magnesium alloy foundry have been led to seek solutions which would resolve it.
The addition of calcium has been found, for example, to reduce the presence of microporosity in the magnesium alloys listed above. British patent no. 847.992 may be quoted in this field. This states on page 2, lines 95-99, that magnesium alloys with a high aluminium and zinc content tend to form microshrinkages and that the presence of calcium greatly reduces the tendency. It may be noted however that according to claim 1 the quantities used are from 0.5 to 3%. These are relatively large quantities and create some difficulties in manufacture, particularly adhesion of the metal and/or pieces to the equipment.